The invention relates to a process for making microfilaments and in addition, the invention is also concerned with a spinning device for making microfilaments.
Synthetic filaments having a single titer of less than 1 dtex are called microfilaments (the term 1 dtex means that 10 km of the thread or filament weighs 1 gram). The microfilaments have, therefore, a very small diameter and are being twisted into microfilament yarns in a known manner. These microfilament yarns can be woven or knitted in order to produce a textile. Due to the single titer of less than 1 dtex, the textiles are distinguished by a very soft touch and an elegant drape so that they have a silk-like character and can join the fashion trend of silk textiles.
Microfilaments are produced by drawing the microfilament at high drawing speeds from a spinning aperture of a spinning nozzle supplied with molten material and drawing it and taking it up on a roll after passing it through an area through which cooling air has been transversely blown. After that, a multitude of microfilaments are twisted to a microfilament yarn from which the desired textile can be produced by weaving.
It is also known to produce spun bond fabric from the microfilaments by drawing the filaments leaving spinning nozzles under the effect of an injector after passing through an area through which cooling air has been transversely blown and depositing them on a constantly moving receiving conveyor Such spun bond fabrics made of microfilaments are also included in the invention. The microfilaments produced from synthetic polymers have a filament diameter, depending on the synthetic polymer used, below 12 .mu.m in polypropylene and below 11 .mu.m in polyamide or below 10 .mu.m in polyester. The mircofilament yarns produced therefrom, which are being offered mostly as polyamide and polyester yarns, as a rule have a single titer which falls only insignificantly below 1 dtex.
As mentioned above, the microfilament yarns and textile products are similar to the fashionably preferred natural silk due to their soft feel. But the textile yarns made of microfilaments have an additional advantage due to the density of the flat structure. Textile fabrics made of microfilament yarns can be woven so densely that they are in their diffusion characteristics similar to semipermeable diaphragms. These flat structures breathe, i.e. they allow easy passage of gases and also vapors, such as water vapor, although at the same time it is very hard to wet them. This low wettability is due to the small filament diameter and the unfavorable angle formed thereby between two filament surfaces.
The advantageous characteristics of the textiles made of filament yarns and also the spun bond fabrics made therefrom can be traced to the relatively small diameter of the microfilaments which are being produced in the manner described further above according to the common "quick spinning process" and are being combined as a rule into "POY-yarns" (POY=partially oriented yarn). The molten polymer material is extruded by the spinning nozzle, cooled underneath the spinning nozzle by an airstream and drawn at high speed--usually about 6,000 m/min.
In order to further increase the silk-like character of the products produced from the microfilaments (textile or spun bond fabric) and to further improve the described advantages even more, commercial operations strive to reduce the diameter of the microfilaments during their production to a single titer of substantially below 1 dtex. Under the usual practical assumption that one should maintain a like total titer of the microfilament yarn also with finer microfilaments, the number of microfilaments in the yarn or the number of nozzle bores per microfilament yarn must increase proportionally to the reduction of the single titer in dtex, since for the production of a microfilament yarn with the same diameter several times the number of microfilaments are needed. In order to obtain the smaller diameters of the microfilaments, it is necessary to reduce the stream of the mass through the unchanged nozzle bore (spinning aperture).
When a process for the manufacture of microfilaments with smaller diameters is commercially realized, it must be also considered, however, that the filament surface at the same volume is conversely proportional to the third power of the filament diameter. For example, if the single titer is halved, then the thinner filament has an eight-fold surface area.
The larger surface area must be seen, however, in connection with the cooling of the microfilament. The extension of the microfilament fundamentally presupposes a certain temperature, and if the cooling is too strong, the danger exists that the microfilament becomes brittle and tears, especially at the usually high drawing speeds of 6,000 m/min.
When the cooling is too fast, an under-cooled skin will be formed on the surface area of the microfilament. This skin is responsible for a breaking of the filaments because the skin is already stiff while the inner mass, which is surrounded by the skin, is still in the extendable condition.
Only by considerably reducing the drawing speed, and at the same time correspondingly decreasing the stream of the molten material through the spinning nozzle, is a remedy here possible. In the other case, when the stream of the mass stays constant, reduction of the drawing speed would have the result that the filaments could not be produced with the desired small diameter.
The corresponding necessary reduction of the drawing speed leads to values of about 2,000 m/min (as compared to the usual value of 6,000 m/min). In connection with the correspondingly reduced stream of the mass, however, a considerably reduced performance of the spinning device results, which is economically not feasible. Since, when realizing a correspondingly dimensioned spinning device with the spinning conditions to be considered, one usually goes as far as to a just acceptable borderline regarding tearing of the filament, the quality of the microfilamentary yarn is additionally affected, not to mention that the economy of a corresponding spinning device suffers so that--in comparison with filaments with larger diameters and with increased drawing speed--fewer yarns can be produced per time unit.
A process for making filaments is known from EP-A-0 244 217 which addresses the problems of the handling of filaments which have been freshly drawn at high drawing speeds from a spinning nozzle and extended. A cylindrical pressure chamber in which the filaments are received immediately after they have been drawn from the spinning aperture is arranged directly underneath the spinning aperture of the spinning nozzle.
Within the pressure chamber there is concentrically arranged a cylindrical sieve, an the pressure chamber is supplied from the outside under pressure with warm air and the warm air is pressed through the cylindrical sieve in which the filaments freshly extruded from the spinning nozzle are drawn. Supply of the warm air takes place in a direction which is predominantly transverse toward the drawing direction of the filaments, whereby the filaments are subjected to a strain within the pressure chamber or within the cylindrical sieve. Also, in the cylindrical sieve turbulences necessarily take place which represent an additional strain for the freshly drawn filaments.
The direction of the warm air runs parallel to the direction of the filaments only subsequently to the pressure chamber, which opens into an exit pipe. This means that the filaments can only be enveloped jacket-like by the warm air after they have left the exit pipe connected below the pressure space. The known method is not suited for the manufacture of microfilaments, i.e. of filaments having a single titer of less than 1 dtex, since the mentioned stresses within the pressure chamber or within the cylindrical sieve, i.e. in an area which is connected directly to the exit opening of the spinning aperture, are too high. Besides, cooling air is not blown transversely against the filaments in a cooling area, but parallel to the direction of the filaments.
EP-A-O 245 011 further shows a similar process for the production of filaments in which the filaments are drawn at high drawing speed through a spinning aperture from a spinning nozzle fed by molten material after passing through a cooling area and are extended. Here, too, a chamber with a cylindrical sieve is connected directly to the spinning aperture, by which sieve warm air under pressure is provided transversely in the direction of the filaments. Only after leaving the named chamber, the warm air supplied runs in a direction parallel to the filaments. Thus, for the area directly att he exit opening of the spinning aperture, The disadvantages mentioned above apply when producing microfilaments with a very small diameter.
It is the object of the invention to provide a process which makes possible the production of microfilaments with very small diameters without lowering the economy and quality. In addition, the invention provides a spinning device which allows economical production of microfilaments with small diameters.